DON’T mud wrestle with a pig, you get dirty and the pig likes it.

By the same token it is a reckless politician who — unless they are very sure of their facts — gets into a statistical standoff with the Institute of Fiscal Studies.

But however magisterial the tone deployed by Paul Johnson of the IFS in discussing the spending plans of the two main contenders in this election (and the LibDem also-rans) we have to step back and look at the politics.